This invention is directed to a new and improved method of manufacture of circuit boards e.g., those having a plurality of through holes interconnecting the various metal layers of a multilayer circuit board.
Multiple layer copper clad circuit boards are used in constructing many electronic products. These circuit boards typically are copper cladding layers e.g., double sided or multilayer, formed into various circuit configurations separated by dielectric material layers. In order to interconnect the copper layers, through holes are provided which extend between the layers. The through holes are electroplated with copper in order to form a conductive pathway between the copper clad layers. In order to facilitate electroplating of the through holes, a thin layer of the conductive polymer film has been used over the board, see U.S. Pat. No. 5,528,000 and 5,415,762 (the entire contents of these patents are incorporated herein by reference hereto). The aforementioned patents should also be referred to for a description of the art. Thereafter, the conductive polymer liquid composition is then dried to remove liquids to form a film thereof. Etchants are then used to remove the conductive polymer positioned over the conductive copper cladding without removing the conductive polymer layer from the dielectric extending into the hole to expose the clad copper layers in the hole prior to electroplating. Such processes have used microetchant compositions which are based among other things on sulfuric acid and hydrogen peroxide combinations, see Printed Circuits Handbook, 4th Edition, Clyde Combs Edition McGraw-Hill .COPYRGT. 1996, Chapter 21, 4, Etching Solutions and in particular Chapter Section 21, 4,2 Sulfuric acid--Hydrogen Peroxide Etchant Systems (the entire contents of this chapter are incorporated herein by reference hereto). Thereafter, the product has been electroplated with copper to form the circuit board.
This present invention provides substantially improved results in comparison with water rinsed polypyrrole conductive polymer systems. In particular the present invention provides improved substrate coverage over glass fiber substrates, longer dwell times in the final etch giving cleaner interconnects and less expensive equipment costs for operating the process. The present invention also provides improved adhesion over graphite systems.
It has now been found that new and improved products are formed by applying certain organic solvent coatings as will be described herein, e.g., by dipping or spraying the organic solvent coating on the conductive polymer suspension already on the copper clad circuit prior to the drying, etching and then electroplating steps to form an electrically conductive film.
In this way, it is now possible to provide circuit boards having substantially fewer interconnect defects due to poor bonding between the copper cladding of the circuits extending to the hole side surface and the electroplated copper.